A PON system is an optical communication system that performs optical splitting in a P2MP (Point to Multi Point) connection mode, with no power and includes a station side device; a single-core optical fiber network where an optical fiber connected to the station side device is split into a plurality of optical fibers through an optical coupler; and home side devices connected to ends of the split optical fibers, respectively.
In the PON system, NRZ (Non-Return to Zero) optical signals obtained by directly or externally modulating a light source such as a semiconductor laser are transmitted, by which information is sent and received.
A downstream optical signal sent from the station side device is transmitted to each home side device in broadcast form. Each home side device performs a reception process on only signals destined therefor. On the other hand, upstream optical signals from the home side devices are managed by the station side device using a time-division multiplexing scheme in order to prevent collisions. The station side device receives the upstream optical signals from the home side devices in a burst manner.
In such a PON system, the larger the number of splits and the transmission distance, the poorer the signal quality. Thus, the PON system is normally operated within 32 splits and within a transmission distance of 20 km.
In view of this, there is proposed a PON multiplexing relay device in which in order to extend the transmission distance between a station side device and home side devices as much as possible, eight G-EPONs are multiplexed into 10G by a pair of relay devices which face each other (see Patent Literature 1).
In addition, as another relay device that achieves the extension of the transmission distance, there is also already known a relay device that recovers and synchronizes an optical signal in G-EPON to an electrical signal and relays the optical signal (see Patent Literature 2).
Furthermore, there is also already known sleep control of an intermittent activation scheme where in order to reduce the power consumption of home side devices in a PON system, a home side device stops communication only for a predetermined sleep time, which is triggered by a sleep instruction from a station side device (see Non-Patent Literature 1).
In this sleep control, the station side device transmits a sleep instruction including information on sleep time to a specific home side device. When a predetermined sleep condition is satisfied, the home side device transitions to a sleep state only for the specified sleep time.